Method and apparatus for testing a fluid conduit system for leaks

ABSTRACT

A method and apparatus for testing a fluid conduit system for leaks. The apparatus is connected to an isolated section of a domestic fuel gas system which is suspected of leaking. The apparatus determines over recorded time periods the pressure drop in the system as a result of the actual gas leak and subsequently the pressure drop in the system as a result of an actual leak plus the leakage through an introduced artificial and calibrated orifice provided by the apparatus. The data is used in an equation for calculating the actual leakage rate from the system. This method avoids the need to repressurize the system between pressure drop measurements or having to know or assume the volume of the system being tested.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for testing a fluidconduit system, such as a domestic fuel gas supply system orinstallation, for gas leaks and, more particularly, for monitoring anddetermining the volumetric leakage rate of gas from such a system orinstallation.

2. Discussion of the Background

The portion of the domestic system with which the applicants areprimarily concerned comprises: the pipework between the gas meter andthe main stop cock or valve (normally at or fairly close to theupstream, inlet side of the gas meter); the gas meter itself; and thepipework between the outlet of the meter and the gas control valve(s) onthe downstream appliance(s). One presently used method for testing suchsystems for leaks where there is a suspected leak involves isolating thesystem by closing the stop valve and the or each gas control valve,pressurising the system by opening the stop valve, closing the stopvalve, and from a predetermined pressure in the system measuring thepressure drop in the isolated system over a set length of time. If apredetermined maximum acceptable pressure drop is not exceeded then thesystem is considered to be sound.

The above described present method assumes that all such system portionshave approximately the same volume. In the past this was a reasonablyvalid assumption to make since the volume of the gas meter has by farcontributed the major part of the total volume of the isolated systemand in the UK a standard domestic gas meter has been employed for manyyears. Consequently, the volume differences in the pipework in differentsystems could be ignored. However, with the introduction of new,relatively low volume gas meters it will no longer be appropriate toignore the differences in the pipework volume from system to system orthe differences between different low volume gas meters and thus theassumption previously made will no longer be a valid one.

A method of testing an industrial gas system or installation todetermine the volumetric leakage rate without having to ascertain orassume volume of the system being tested is known already. The methodinvolves isolating the system, pressurising the system to apredetermined pressure, measuring the pressure drop (ΔP₁) in the systemfrom that predetermined pressure over a fixed time period (T₁),repressurising the system to the same starting or predetermined pressureand introducing a calibrated orifice, i.e. a known `artificial leak`,into the system, and then measuring the time period (T₂) over which thesame pressure drop (ΔP₁) occurs with the system leaking as a result ofboth the actual leak and the `artificial leak`.

The leak rate of the installation is obtained from the equation:##EQU1## where F is the known flow rate through the artificial leakmeasured at the same pressure.

The above described methods are carried out manually employing aso-called `U gauge` manometer to measure the pressure drops and a stopwatch to measure the time periods.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method for measuring leakrate which avoids having to repressurise the system and apparatus forcarrying out the method.

According to one aspect of the invention, there is provided a method ofdetermining the rate of leakage of fluid from a leaking fluid containingsystem, the method comprising pressurising the system, terminating thepressurisation, monitoring from a start time (T_(o)) and pressure(P_(o)) a first drop in pressure in the system and the time, determiningthe time taken (T₁) for the pressure in the system to drop by apredetermined pressure drop (ΔP₁) to pressure P₁, or where ΔP₁ has notbeen reached in a time less than a predetermined time (T₂), determiningpressure drop (ΔP₂) to pressure P₁ over the predetermined time T₂, andthen, without repressurising the system, monitoring from a new starttime which substantially coincides with the end of the time period (T₁)or the predetermined time period (T₂), the time taken (T₃) for a seconddrop in the pressure in the system (ΔP₃) through the actual leak in thesystem and a calibrated leak introduced into the system combined, whereΔP₃ =(ΔP₁ or ΔP₂)×K, whichever of ΔP₁ and ΔP₂ was determined initiallybeing applicable and K being a constant, and calculating the actualleakage rate according to the following formula ##EQU2## where C is aconstant.

Constant K, referred to above may be equal to 1, in which case ΔP₃ =ΔP₁or ΔP₂, as appropriate.

However, Applicants have found that more accurate leak rate measurementresults can be obtained if the leak rate equation takes account (atleast to some extent) of the fact that the second pressure drop startsfrom a lower pressure than the first pressure drop and that thedischarge rate through the actual or real leak will thus be reducedduring the second pressure measurement compared with the position if ithas been measured compared with the position if it had been measured atthe initial higher starting pressure (P_(o)). More specifically,Applicants have obtained such more accurate results by requiring thesecond pressure drop ΔP₃ over which T₃ is to be determined to havesubstantially the same fractional change as the first pressure drop ΔP1or ΔP₂ ; in which case, in the equation:

    ΔP.sub.3 =(ΔP.sub.1 or ΔP.sub.2)×K ##EQU3## The method may be employed in monitoring a fuel gas system comprising a gas meter, pipework between the gas meter and a stop cock or valve located upstream of the inlet to the meter, and pipework between the outlet of the meter and the gas control valve(s) of one or more gas-fired appliances located downstream of the meter. In such a case the method includes, prior to pressurising the system, closing the stop valve and the or each gas control valve, pressurising the system by opening the stop valve to allow gas into the system, and then closing the stop valve to produce an isolated system.

After terminating pressurisation of the system and prior to monitoringthe first drop in pressure in the system, the pressure in the system mayconveniently be checked to ensure that the pressure is no longer risingbefore initiating the monitoring of the first drop in pressure in thesystem. If the check indicates that the pressure in the system is stillrising, this usually means one of two things. Firstly, that the stopvalve is faulty and is still allowing gas from the supply source past itinto the system to be monitored. In such a case the stop valve should bereplaced before the test can be carried out for gas leakage from thesystem. Secondly, that the system has not yet stabilised. In which casethe start time of the period over which the first pressure drop in thesystem is measured is delayed until the pressure in the system hasstabilised. In the present context `stabilisation` means the situationwhere the pressure in the system is either constant or falling.

However, where the system incorporates a gas pressure regulator orgovernor between the stop cock and the inlet to the meter, the pressuremay initially rise on closing the stop cock before it starts to fall.This may be caused by so-called governor lock-up where the governorprevents release of further gas to the downstream part of the system.This occurs when the meter governor is designed to `lock-up` if thedownstream gas pressure exceeds a predetermined maximum. It is possiblethat the part of the system downstream of the governor could bepressurised to a pressure greater than the predetermined allowablemaximum if the line pressure upstream of the stop cock exceeds thisvalue and the downstream part of the pipework is initially pressurisedby rapidly opening and closing the stop clock. The governor would thenlock up. As the system leaks and the pressure drops the governor wouldthen unlock and release gas from the pipework between the stop cock andthe governor to the downstream part of the system. The pressure in theisolated downstream part of the system could then initially increaserather than decrease.

According to another aspect of the invention, there is providedapparatus for use in determining the rate of leakage of fluid from afluid containing system, the apparatus comprising:

inlet means for receiving fluid from the system,

conduit means connecting the inlet to fluid pressure sensing means,

valve means selectively openable to connect a calibrated orifice meansto the conduit means,

data storage means,

data processing means,

data display means,

programmed control means, and

means operable to initiate, from a start time and with the valve meansclosed, the control means to monitor drop in pressure in the system, asmeasured by the pressure sensing means, with time to establish if apredetermined pressure drop ΔP₁ occurs in a time T₁ less than apredetermined time T₂, and if so to store data representing ΔP₁ and T₁in the data storage means, but if ΔP₁ is not reached by predeterminedtime T₂ to store in the data storage means data representing T₂ and thepressure drop ΔP₂ which has occurred by time T₂ ; and then from a newstart time, which substantially coincides with the time T₁ (where ΔP₁has been stored) or time T₂ (where ΔP₂ has been stored), to open thevalve means, whereby when the apparatus is in use the calibrated orificeis placed in communication with the fluid conduit and thus with thesystem via the inlet, and to establish the time taken T₃ for thepressure as sensed by the pressure sensing means to drop by ΔP₁ (whereΔP₁ has been stored) or by ΔP₂ (where ΔP₂ has been stored), and to storedata representing T₃ in the data storage means; and to input into thedata processing means from the data storage means data representing T₁or T₂ and T₃, the data processing means being programmed to calculatethe rate of leakage of the system using the formula, leak rate= ##EQU4##where C is a constant; and to cause the calculated leak rate to bedisplayed on the data display means.

According to a further aspect of the invention, there is providedapparatus for use in determining the rate of leakage of fluid from afluid containing system, the apparatus comprising:

inlet means for receiving fluid from the system,

conduit means connecting the inlet to fluid pressure sensing means,

valve means selectively openable to connect a calibrated orifice meansto the conduit means,

data storage means,

data processing means,

data display means,

programmed control means, and

means operable to initiate, from a start time and start pressure P_(o),with the valve means closed, the control means to monitor drop inpressure in the system, as measured by the pressure sensing means, withtime to establish if a predetermined pressure drop ΔP₁ to pressure P₁occurs in a time T₁ less than a predetermined time T₂, and if so tostore data representing ΔP₁ and T₁ or ΔP₁, P₁ and T₁, in the datastorage means, but if ΔP₁ is not reached by predetermined time T₂ tostore in the data storage means data representing T₂ and the pressuredrop ΔP₂ to pressure P_(L) which has occurred by time T₂, or ΔP₂, P_(L)and T₂ ; and then from a new start time, which substantially coincideswith the time T₁ (where ΔP₁ has been stored) or time T₂ (where ΔP₂ hasbeen stored), to open the valve means, whereby when the apparatus is inuse the calibrated orifice is placed in communication with the fluidconduit and thus with the system via the inlet, to input into the dataprocessing means from the data storage means data representing ΔP₁ orΔP₂, whichever has been stored, the data processing means beingprogrammed to calculate a second pressure drop ΔP₃ in the system to besensed by the pressure sensing means using the formula ΔP₃ =(ΔP₁ orΔP₂)×K, wherein K is a constant, to store data representing ΔP₃ in thedata storage means, and to establish the time taken T₃ for the pressureas sensed by the pressure sensing means to drop by ΔP₃, and to storedata representing T₃ in the data storage means; and to input into thedata processing means from the data storage means data representing T₁or T₂ and T₃, the data processing means being programmed to calculatethe rate of leakage of the system using the formula, leak rate= ##EQU5##where C is a constant; and to cause the calculated leak rate to bedisplayed on the data display means. ##EQU6## In such a case theapparatus data storage means stores data representing start pressureP_(o), and from the data storage means data representing P_(o) and ΔP₁or ΔP₂ is input into the data processing means which is programmed tocalculate K according to the formula ##EQU7## for use in the formula forcalculating ΔP₃. Alternatively, in addition to data representing P_(o)being stored, data representing P₁ and P_(L) is stored in the datastorage means, and from the data storage means data representing P_(o),P_(l) and P_(L) is input into the data processing means which isprogrammed to calculate K according to the formula ##EQU8## for use incalculating ΔP₃.

The control means may be programmed so that the apparatus does not startto monitor pressure drop in the system unless the existing pressure isgreater than a predetermined pressure. In such a case the display leakrate determining process is terminated, and means may be caused toindicate that the pressure must be above the predetermined pressure. Theuser of the apparatus may then decide to pressurise the system furtherby opening the stop cock again and then initiating the leak ratedetermination process again.

The control means may also be programmed to determine (a) if T₁ is lessthan a predetermined minimum time T_(min) and if so to cause the leakrate determining process to be terminated and to cause the display meansto indicate that the leak is too large to be measured accurately by theapparatus; and (b) if ΔP₂ is less than a predetermined minimum pressuredrop ΔP_(min) and if so to cause the leak rate determining process to beterminated and to cause the display means to indicate that the leak istoo small to be measured accurately by the apparatus.

In addition the control means may be programmed to determine if T₃ islarger than T₁ or T₂ (whichever is applicable) and if so to cause theleak rate determining process to be terminated and to cause the displaymeans to indicate that there is an error.

The programmed control means may also be operable by initiating means,as an alternative to the leak rate monitoring programme described, tocause pressure of the fluid being sensed by the pressure sensor to bedisplayed by the data display means. Such a facility may be used tocheck that the pressure in the system is not rising before initiatingthe programmed control unit to determine leak rate.

The programmed control means may further also be operable by initiatingmeans to monitor drop in pressure ΔP_(x) in the system, as measured bythe pressure sensing means, from a start time over a predetermined timeperiod T_(y), and causing the data display means to display, at the endof the predetermined time period T_(y), both ΔP_(x) and T_(y).

Between the start time and time T_(y) the display means may display atregular time intervals the current pressure and the time remainingbefore T_(y) is reached, for example on a second to second intervalbasis.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic view of part of a domestic fuel gas system,

FIG. 2 is a schematic view of the front panel and interior of oneapparatus according to the invention,

FIG. 3 is another schematic view of the apparatus, and

FIGS. 4a, 4b, 4c, 4d, 4e, 4e' and 4f shows a flow chart of operatingsequences for the apparatus of FIGS. 2 and 3.

DISCUSSION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates part of a domestic fuel gas system orinstallation as an example of the kind of fluid containing system onwhich the method and apparatus may be used in order to determine theleak rate of fluid escaping from the system.

The system comprises a gas supply pipe 1 connected to a gas main (notshown). A stop cock 2 is provided for closing off the pipe andpreventing supply of gas to downstream gas-fired appliances 3 and 4 viagas meter 5.

A gas regulator or governor 6 is provided between the stop cock 2 andthe gas meter 5. The gas-fired appliances 3 and 4 have associatedtherewith gas control valves 7, 8 and 9 via which gas can be preventedfrom passing through the appliances. The gas meter 5 has a normallyclosed tapping hole 10 via which access may be had to the interior ofthe system on using an appropriate tool or instrument to manipulate thetapping hole to an open position.

The apparatus 20 schematically illustrated in FIGS. 2 and 3 comprisesupper and lower body parts 22a and 22b which form a housing 22 whichmay, conveniently be sized, to be held in the hand.

The lower body part 22b includes an inlet means 24 via which theapparatus can be connected to the tapping hole 10 by means of, forexample, a length of flexible tubing 26 (see FIG. 1). A fluid conduit 28connects the inlet 24 to fluid pressure sensing means 30 whichpreferably is of a kind which compensates at least to some extent forchanges in temperature. A valve means 32, which in this example is alatching solenoid valve, is selectively operable to connect a calibratedorifice 34 to the conduit means 28 via line 36. The calibrated orifice34 can be connected to a vent 38 to atmosphere via line 40. The pressuresensing means 30 is connected to an atmospheric reference port 42 toprovide gauge pressure, or this could be sealed to provide adifferential or absolute pressure reading, via line 44. The pressuresensing means 30 is connected to a micro-controller 50 via an amplifier52. The micro-controller enables various programmed operations to becarried out automatically once the appropriate programme has beeninitiated. The micro-controller 50 is also connected to: A ROM 54 thefunction of which here is to hold the read only computer programme; aRAM 56, one function of which here is to store the measured variables ofpressure and time; a relay 58 which connects the micro-controller to thesolenoid valve 32; and a data display means 60 in the form of LCD whichis located on the front panel 22a.

It will be appreciated that the micro-controller 50 includes dataprocessing means for performing mathematical calculations in accordancewith the programmed control.

Input signals for instructing the micro-controller and initiating thechosen programme (as will be described below) are achieved by operatingbuttons 62, 64, 66 located on the front panel.

To conserve battery power the light for the display may be programmedsuch that it remains "on" for a limited time period only.

A Ni Cad battery 72 is used for supplying electrical energy to theinstrument. The Ni Cad battery may be recharged through an externalbattery charging arrangement, for example by means of a household mainspower supply via connector/socket 74.

FIGS. 4a, 4b, 4c, 4d, 4e, and 4e' show a flow chart of operating andprogrammed control sequences. In FIG. 4, as shown by the key, the plainboxes are automatically controlled operations and those with doubleedging are manual operations. Going through the chart in sequence, atthe start 100 a manual check is made at stage 101 to ensure that thestop cock 2 and the appliance valves 7, 8 and 9 are closed. The inlet 24of the apparatus 20 is connected to the system at the tap-in-point 10 onthe gas meter 5 by means of the flexible tubing 26--stage 102. Thepower, for example from the battery 72, is switched on at stage 103 andthis is indicated by the LCD 60 being activated. Optionally, at thisstage the display means 60 may inform the user to open the gas inletmeans 24 to atmosphere. This will provide equal pressures to both sidesof the pressure sensor enabling a software routine (not shown) for thepressure sensor 30 to zero itself, whereafter the button 64 may bepressed to clear the display. At stage 104 the system between the stopcock 2 and the valves 7, 8 and 9 is pressurised by opening the stop cockand then closing it. After this the `menu stage` 105 is reached at whichthere are two options: Option A--to view the current pressure as itchanges with time, for example on a second to second basis, and then, ifdesired, to initiate a control programme for determining the pressurechange in the system over a predetermined time period; and Option B--toinitiate a control programme for determining leak rate of gas from thesystem.

Option A will be described first. When button 62 is pressed the displaymeans 60 continually displays the pressure of the isolated system--stage106. From stage 106, if desired, button 64 may be pressed in order forthe apparatus to measure from a start time pressure drop in the systemover a predetermined period, for example, 2 minutes--stage 107.Optionally, a `stabilisation` time period may be introduced prior tocommencement of the measurement of the pressure drop. The purpose of the`stabilisation` time period here is to avoid pressure fluctuations dueto, for example, temperature or governor effects. During thepredetermined time period the display means displays the existingpressure and the time remaining before the predetermined time isreached--stage 108. At the end of the predetermined time the displaymeans displays predetermined time period and the pressure drop that hasbeen measured in the predetermined period--stage 109. At any of stages106, 108 and 109, the procedure may be returned to stage 105, i.e. themenu stage by pressing button 62.

Option B, which may be followed from stage 105 by pressing button 64,will now be described. On pressing button 64, the apparatusautomatically determines if the current pressure in the system isgreater than a predetermined maximum pressure--stage 110. If yes, thesolenoid valve automatically opens to place the artificial calibratedorifice in communication with the fluid conduit until the pressure fallsto a predetermined maximum pressure whereat the solenoid valveautomatically closes the calibrated orifice--stage 111, from which theprogramme proceeds to stage 112 whereat the display means displays thepressure and to wait a predetermined `stabilisation` time period. At theend of the `stabilisation` time period the display means displays thecurrent pressure and "go when ready" to complete stage 112. If at stage110 the pressure is not greater than a predetermined maximum pressure,the programme proceeds to stage 112 as described above. Following stage112 a check is made at stage 113 to see if the display means indicatesthat the pressure is falling or starting to fall, and if it does thenwhen the pressure has fallen to below a predetermined value the button64 may be pressed again to initiate the leak rate determiningprogram--stage 114.

If the pressure check at stage 113 indicates that the pressure is notfalling but is constant this may indicate that the system is not leakingor, and perhaps less likely, that the stop cock and/or regulator is/areallowing upstream gas into the system at the same rate that a leak inthe system is allowing gas to leak from it. If the pressure check showsthat the pressure is still rising, even after the stabilisation period,for example of 1 minute, this indicates that the stop cock is leakingand still allowing gas from the gas main into the portion of the systemunder test. As indicated earlier the stop cock would have to be replacedbefore the test is carried out. The apparatus may be programmed toindicate that the stop cock may require replacement. Since gas from thesystem and air have different flow rates through any given opening oraperture, any air present in the tubing 26 from the meter to theapparatus which flows through the calibrated orifice instead of gascould result in an incorrectly determined leak rate. To reduce oralleviate this potential problem the apparatus may be operated manuallyor automatically so as to cause the latching solenoid valve to open for,say 10 seconds during the stabilisation period in order to flush some ofthe air out of the tubing 26 and through the calibrated orifice 34.

Stages 110 and 111 accommodate the possibility that the pressure mayinitially rise on closing the stop cock 2 before beginning to fall, forexample as a result of governor lock-up as described earlier. Thus, theapparatus may be designed so that the latching solenoid valvecontrolling the opening/closing of the artificial leak or calibratedorifice automatically opens the orifice if the pressure in thedownstream part of the system exceeds a predetermined pressure with theorifice remaining open until the pressure falls to a value below thepredetermined pressure, whereat the solenoid valve then closes theorifice. Thereafter, optionally, the programme may proceed from stage111, via an automated route to stage 115 where the leak rate measurementdetermining programme is initiated. This provides a fast method ofreducing pressure in the system following governor lock-up.Alternatively, the tubing may be manually purged with gas by the userprior to measurements being taken by the apparatus, and to facilitatethis the tubing and the apparatus may be provided with quick-releaseco-operating couplings.

Assuming that stage 114 (or 115) is reached, then the apparatusautomatically determines if the current pressure in the system isgreater than a predetermined minimum pressure--stage 116. If it is not,the display means displays that the pressure of the system must be abovethe predetermined minimum pressure and is an indication that the leakrate determination process was initiated with the system pressure toolow--stage 117.

At any of stages 112, 113 or 117 the process may be returned to the menustage 105, and the preceding pressurising stage may be repeated, forexample to raise the pressure of the system sufficiently for it to begreater than the predetermined minimum pressure at stage 116.

Thus, providing the pressure is greater than the predetermined pressureat stage 116 the programme proceeds to stage 118 whereat the time is setas the `start time` and the pressure, P_(o), at the start time is storedin RAM 56, and then the display means indicates that the leak rate testis in progress by, for example, displaying a lighted LED or theinformation "wait . . . test in progress"--stage 119. Following stages118 and 119, the apparatus automatically determines if a predeterminedpressure drop ΔP₁ has occurred in a time T₁ which is less than apredetermined time T₂ --stage 120. If yes, it is then determined if T₁is less than a predetermined minimum time T_(min) --stage 121: if yes,the display means indicates that the leak is too large to be measuredand the test is terminated--stage 122; whereas, if no, P₁, ΔP₁ and T₁are stored and the time is reset from the time T₁ --stages 123 and 124.If at stage 120, ΔP₁ has not occurred in a time less than time T₂, thepressure drop ΔP₂ that occurs in time T₂ is determined--stage 125. It isthen determined if ΔP₂ is less than predetermined minimum pressure dropΔP_(min) --stage 126: if yes, the display means indicates that the leakis too small to be measured and the test is terminated--stage 127;whereas, if no, P₁, ΔP₂ and T₂ are stored and the time is reset from thetime T₂ --stages 128 and 129.

Coincident with stage 124 or 129, at the reset time T₁ or T₂, asapplicable, the solenoid valve 32 is automatically operated to place thecalibrated orifice in communication with the fluid conduit so that thesystem is leaking from both the actual leak in the system and theintroduced calibrated leak--stage 130. Using ΔP₁ and P₁, or ΔP₂ andP_(L), as applicable, and P_(o), ΔP₃ is calculated at stage 131according to the formula shown. The time taken T₃ from the reset timefor the pressure drop ΔP₃ to occur is then determined and stored--stage132.

At stage 133 it is determined if T₃ is greater than T₁ or T₂ (asapplicable) : if yes, the display means indicates that there is an errorand the test is terminated--stage 134; if no, T₃ is stored--stage 135.Using T₁ or T₂, and T₃, the leak rate of the actual leak is calculatedat stage 136 according to the formula: ##EQU9## The display means thendisplays the calculated leak rate--stage 137.

By way of illustration only, in one of the system portions of a gasinstallation tested, using the method and apparatus according to theinvention, the volume of the isolated system, including the gas meterand associated governor was approximately 10 liters. The system waspressurised to about 20 mbar and the start of the leak rate test wasdelayed (stabilisation period) for about 1 minute. The predeterminedpressure above which the pressure in the isolated system had to be atstage 113 for the test to continue to stage 115 was 18 mbar. At stage117, the predetermined ΔP₁ was 1.5 mbar and predetermined time T₂ was 20seconds. At stage 126, the predetermined minimum pressure drop ΔP_(min)was 0.0668 mbar. At stage 136, the calibrated leak rate of the orificeused was 66 ml/min and the constant C used was 0.935.

The figure of 0.935 used as the constant was derived from experimentalresults on a wide range of system volumes and leak rates. The reasonthat the constant is not "1" is because the leakage of fluid through anorifice is not ideally proportional to pressure drop.

For total leak rates of less than 200 ml/min through leaks in the actualsystem, the results provided by the device were within about ±3% of theresults when the leak rates were determined by a standard bubble flowmeter measurement method.

Thus Applicants have found that when the actual leak in the system beinginvestigated is between being somewhat smaller than, and not too muchgreater than, the calibrated leak then the hand-held apparatus employedin the test described above was simple to use and gave quick and fairlyaccurate leak rate measurements when compared with referencemeasurements obtained by much slower and less convenient methods knownto provide accurate results.

It will be appreciated that this readily portable apparatus allowsengineers to carry out service checks on domestic gas installations incustomers homes in relatively little time.

Whilst a particular embodiment of the invention has been describedabove, it will be appreciated that various modifications may be madewithout departing from the scope of the invention. For example, thefront panel may have a different number and or arrangement of operatingbuttons for pressing to initiate the different functions or programmesof the apparatus. Also, there may be small LED's associated with eachbutton on the panel to indicate to the user of the apparatus whichfunction, e.g. leak rate determining mode, is currently in operation.The display may simply provide the pressure readings or leak rates.Furthermore, the battery for supplying power to the apparatus need notbe contained within the housing 22 but may be incorporated in a separateplug-in unit or device which is plugged into the housing when required.

It would also be possible particularly in a fully automated operationalsequence to replace the menu driven procedure by a simple display of thepressure and time values and a series of LED indicators showing thevarious stages of the test routine.

What is claimed is:
 1. A method of determining the rate of leakage offluid from a leaking fluid containing system, the method comprisingpressurising the system, terminating the pressurisation, monitoring froma start time (T_(o)) and pressure (P_(o)) a first drop in pressure inthe system and the time, determining the time taken (T₁) for thepressure in the system to drop by a predetermined pressure drop (ΔP₁) topressure P_(l), or where ΔP₁ has not been reached in a time less than apredetermined time (T₂), determining pressure drop (ΔP₂) to pressureP_(L) over the predetermined time T₂, and then, without repressurisingthe system, monitoring from a new start time which substantiallycoincides with the end of the time period (T₁) or the predetermined timeperiod (T₂), the time taken (T₃) for a second drop in the pressure inthe system (ΔP₃) through the actual leak in the system and a calibratedleak introduced into the system combined, where ΔP₃ =(ΔP₁ or ΔP₂)×K,whichever of ΔP₁ and ΔP₂ was determined initially being applicable and Kbeing a constant, and calculating the actual leakage rate according tothe following formula ##EQU10## where C is a constant.
 2. A method asclaimed in claim 1, in which K=1.
 3. A method as claimed in claim 1, inwhich ##EQU11##
 4. A method as claimed in claim 1, in which C=0.935. 5.A method as claimed in claim 1, in which after terminatingpressurisation of the system the pressure in the system is checked toensure that the pressure is no longer rising before progressing. 6.Apparatus for use in determining the rate of leakage of fluid from afluid containing system, the apparatus comprising: inlet means forreceiving fluid from the system,conduit means connecting the inlet tofluid pressure sensing means, valve means selectively openable toconnect a calibrated orifice means to the conduit means, data storagemeans, data processing means, data display means, programmed controlmeans, and means operable to initiate, from a start time and with thevalve means closed, the control means to monitor drop in pressure in thesystem, as measured by the pressure sensing means, with time toestablish if a predetermined pressure drop ΔP₁ occurs in a time T₁ lessthan a predetermined time T₂, and if so to store data representing ΔP₁and T₁ in the data storage means, but if ΔP₁ is not reached bypredetermined time T₂ to store in the data storage means datarepresenting T₂ and the pressure drop ΔP₂ which has occurred by time T₂; and then from a new start time, which substantially coincides with thetime T₁ (where ΔP₁ has been stored) or time T₂ (where ΔP₂ has beenstored), to open the valve means, whereby when the apparatus is in usethe calibrated orifice is placed in communication with the fluid conduitand thus with the system via the inlet, and to establish the time takenT₃ for the pressure as sensed by the pressure sensing means to drop byΔP_(l) (where ΔP₁ has been stored) or by ΔP₂ (where ΔP₂ has beenstored), and to store data representing T₃ in the data storage means;and to input into the data processing means from the data storage meansdata representing T₁ or T₂ and T₃, the data processing means beingprogrammed to calculate the rate of leakage of the system using theformula, ##EQU12## where C is a constant; and to cause the calculatedleak rate to be displayed on the data display means.
 7. Apparatus asclaimed in claim 6, in which the control means is also programmed sothat the apparatus does not start to monitor pressure drop in the systemunless the existing pressure is greater than a predetermined minimumpressure.
 8. Apparatus as claimed in claim 7, in which when the existingpressure is not greater than the predetermined minimum pressure, displaymeans indicate that the pressure must be above the predetermined minimumpressure.
 9. Apparatus as claimed in claim 6, in which the control meansis also programmed so that the apparatus does not start to monitorpressure drop in the system unless the existing pressure is not greaterthan a predetermined maximum pressure.
 10. Apparatus as claimed in claim9, in which when the existing pressure is greater than the predeterminedmaximum pressure, control means causes the valve means to open and placethe calibrated orifice in communication with the fluid conduit means sothat the pressure can fall to a predetermined maximum pressure whereuponthe control means causes the valve means to close and terminate suchpresent communication between the fluid conduit means and the calibratedorifice.
 11. Apparatus as claimed in claim 6, in which the control meansis also programmed to determine (a) if T₁ is less than a predeterminedminimum time T_(min) and if so to cause the leak rate determiningprocess to be terminated and to cause the display means to indicate thatthe leak is too large to be measured accurately by the apparatus; and(b) if ΔP₂ is less than a predetermined minimum pressure drop ΔP_(min)and if so to cause the leak rate determining process to be terminatedand to cause the display means to indicate that the leak is too small tobe measured accurately by the apparatus.
 12. Apparatus as claimed inclaim 6, in which the control means is also programmed to determine ifT₃ is larger than T₁ or T₂ (whichever is applicable) and if so to causethe leak rate determining process to be terminated and to cause thedisplay means to indicate that there is an error.
 13. Apparatus asclaimed in claim 6, in which the control means is also programmed andoperable by initiating means, to cause pressure of the fluid beingsensed by the pressure sensor to be displayed by the data display means.14. Apparatus as claimed in claim 6, in which the control means is alsoprogrammed and operable by initiating means to monitor drop in pressureΔP_(x) in the system, as measured by the pressure sensing means, from astart time over a predetermined time period T_(y), and to cause the datadisplay means to display, at the end of the predetermined time periodT_(y), both ΔP_(x) and T_(y).
 15. Apparatus as claimed in claim 14, inwhich between the start time and time T_(y) the display means displaysat regular time intervals the current pressure and the time remainingbefore T_(y) is reached.
 16. Apparatus as claimed in claim 6, in whichthe valve means is operable so as to be openable and closable withrespect to atmosphere so that fluid from the system which enters throughthe inlet means can flush through the conduit means and the calibratedorifice means prior to the pressure drop in the system being monitored.17. Apparatus for use in determining the rate of leakage of fluid from afluid containing system, the apparatus comprising:inlet means forreceiving fluid from the system, conduit means connecting the inlet tofluid pressure sensing means, valve means selectively openable toconnect a calibrated orifice means to the conduit means, data storagemeans, data processing means, data display means, programmed controlmeans, and means operable to initiate, from a start time and startpressure P_(o), with the valve means closed, the control means tomonitor drop in pressure in the system, as measured by the pressuresensing means, with time to establish if a predetermined pressure dropΔP₁ to pressure P_(l) occurs in a time T_(l) less than a predeterminedtime T₂, and if so to store data representing ΔP₁ and T₁ or P_(l), P_(l)and T_(l), in the data storage means, but if ΔP₁ is not reached bypredetermined time T₂ to store in the data storage means datarepresenting T₂ and the pressure drop ΔP₂ to pressure P_(L) which hasoccurred by time T₂, or ΔP₂, P₁ and T₂ ; and then from a new start time,which substantially coincides with the time T₁ (where ΔP₁ has beenstored) or time T₂ (where ΔP₂ has been stored), to open the valve means,whereby when the apparatus is in use the calibrated orifice is placed incommunication with the fluid conduit and thus with the system via theinlet, to input into the data processing means from the data storagemeans data representing ΔP₁ or ΔP₂, whichever has been stored, the dataprocessing means being programmed to calculate a second pressure dropΔP₃ in the system to be sensed by the pressure sensing means using theformula ΔP₃ =(ΔP₁ or ΔP₂)×K, wherein K is a constant, to store datarepresenting ΔP₃ in the data storage means, and to establish the timetaken T₃ for the pressure as sensed by the pressure sensing means todrop by ΔP₃, and to store data representing T₃ in the data storagemeans; and to input into the data processing means from the data storagemeans data representing T₁ or T₂ and T₃, the data processing means beingprogrammed to calculate the rate of leakage of the system using theformula, ##EQU13## where C is a constant; and to cause the calculatedleak rate to be displayed on the data display means.
 18. Apparatus asclaimed in claim 17, in which ##EQU14## and the apparatus data storagemeans stores data representing start pressure P_(o), and from the datastorage means data representing P_(o) and ΔP₁ or ΔP₂ is input into thedata processing means which is programmed to calculate K.
 19. Apparatusas claimed in claim 17, in which ##EQU15## and the apparatus datastorage means stores data representing start pressure P_(o), pressureP_(l) and pressure P_(L), and from the data storage means datarepresenting P_(o), P_(l) and P_(L) is input into the data processingmeans which is programmed to calculate K.